What is the default size unit in linux ls -l command

That size is in bytes.

You can use ls -lh to print the long listing with human readable file sizes.

We need to add l(long listing option) to show human readable file sizes (ls -lh). In your case, size of file ser_cat is in 39870 bytes.

-rw-rw-r--    1 guest    guest       39870 Feb 14 19:41 ser_cat
-rw-r--r--    1 guest    guest       19935 Feb 14 19:35 ser_cp
-rw-rw-r--    1 guest    guest       19935 Feb 14 19:29 ser_more

ls -lh command shows all file size information as K for Kibibyte (KiB), M for Mebibyte (MiB) and so on.. Instead of bits they show information in bytes.

ls -lh shows unit (size) information using single character instead of two characters. If no unit information is there, then that is bytes.